I have an aluminum single-speed frame that was marketed as a fixie. I'm wanting to convert it to a single-speed commuter. (I'm getting older and the fixie doesn't agree with my knees anymore.) It has enough clearance in the rear for 700x35 tires. It has a brake bridge but it doesn't have posts to mount v-brakes or cantilevers. Is there any kind of adapter that could make that work?

I would not depend on such things, even if they were available. Why do you not want to use caliper brakes? There are models available that can accommodate wide tires. You may have to deflate the tire slightly to get it through the caliper, but thats much better than having some shoddy adapter come off when you are trying to brake hard.
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whatsisnameMar 29 '13 at 5:34

There are a couple of reasons, more than I can fit into a comment really. But as I said in a comment below, I don't think an adapter would necessarily be "shoddy." I've actually seen frames with movable brake posts so that the frame could be used for 24" or 26" wheels.
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jimirings♦Mar 29 '13 at 20:42

5 Answers
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In fact I used one on my old folding bike - but you can see it looks a bit of a mess, and when I tried it on the front fork, it really wasn't up to the job (I ended up using a chunky long reach caliper brake taken off a BMX bike). (I'd changed wheel size and was using much fatter tyres, so the original brakes wouldn't fit.)

Thank you! That one on Danscomp was exactly what I was envisioning. While the one on Ratrodbikes certainly looks better, I'd feel a little nervous about it since the posts could conceivably roll around the chainstays.
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jimirings♦Apr 3 '13 at 14:52

Great find! Most of those look a little sketchy, but the bolt on type shown on ratrodbikes.com actually look like a real option if they fit your frame. No idea they existed. Deleting my answer because it's obviously wrong.
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joelmdevApr 6 '13 at 16:30

It was definitely sketchy on the front, but seemed to work okay on the back. Part of that might be that braking force was forcing it into the chainstays but away from the fork (the hose clamps aren't my workaround, they came with it as the manfacturer's recommendation), part just that the front brake does more work when braking hard. Having a "brake booster" type U to stiffen it might have helped on the front too, but then it gets even more of a mess. On a standard bike "swap the forks" is an easier solution.
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armbApr 8 '13 at 11:47

I do not think anything bolted on could be good enough. Since it is aluminum frame, brazing or welding the bosses would also be complicated.

But instead of replacing the whole frame, you might just change front fork with one that has bosses. That way on front where it makes more difference you would have V-brake, and on the rear you could have long reach caliper brake.

I don't see why something bolted on couldn't work. If it bolted onto the existing brake bridge and both seat stays, it'd actually be more secure than the way disc brake calipers bolt onto the frame since the braking force would push the adapter against the frame as opposed to the shear force on the bolts that attach disc brake calipers.
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jimirings♦Mar 29 '13 at 20:30

true I was not precise. I am not concerned by the bolting per se. What makes me doubt it in this case is the length from the bolt and the place where force is applied.
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Davorin RuševljanMar 30 '13 at 23:46

I was envisioning something shaped like an upside-down U. The bottom of the U would be bolted onto the brake bridge, and open ends of the U would be bolted onto the seat stays just below the brake posts. But if it doesn't exist I guess it doesn't matter.
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jimirings♦Mar 31 '13 at 17:34

There used to be castings that were designed to fit over existing brake studs and stiffen frames that had too much side-to-side flex for the cantis to be effective. Don't know if they could be adapted to your purpose, though, and don't know if they're still available.

That sounds like it's probably at least the basic idea of what I was envisioning. I have a shop in town that does a lot of work with older bikes. I'll ask them if they know anything about that. Thanks!
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jimirings♦Mar 29 '13 at 20:45

But, wouldn't it be easier to mount side-pull caliper brakes? You can buy a pair of good-quality dual pivot side-pull calipers for $40-$60, and they will just mount on the single-bolt brake bridge you already have.